Microplastics — tiny particles of plastic found in the air, water, soil, and even in the bloodstreams of animals and people around the world — are recognized as a serious pollution threat.
Some microplastics are intentionally added to a variety of products — amounting to an estimated 50,000 tons a year in the EU alone, according to the European Chemicals Agency. The EU has already declared that these nonbiodegradable microplastics must be eliminated by 2025, but a team of scientists at MIT and elsewhere has perhaps expedited that timeline.
They’ve developed a system based on silk that could provide an inexpensive, easily manufactured substitute. The microplastics widely used in industrial products generally protect a specific active ingredient(s) from being degraded by exposure to air or moisture, until they’re needed. They provide a slow release of the active ingredient(s) for a certain period of time and minimize adverse effects to the surroundings.
The European Chemical Agency has estimated that the intentionally added microplastics represent approximately 10-15 percent of the total amount in the environment, but this source may be relatively easy to address using the nature-based biodegradable replacement, said MIT Professor of Civil and Environmental Engineering Benedetto Marelli.
“We cannot solve the whole microplastics problem with one solution that fits them all,” Marelli said. “Ten percent of a big number is still a big number … We’ll solve climate change and pollution of the world one percent at a time.”
While silkworm cocoons must be unwound to produce the fine threads needed for fabric, for this practicality, non-textile-quality cocoons can be used, and the silk fibers can be dissolved using a scalable water-based process. The processing is so simple and tunable that the resulting material can be adapted to work on existing manufacturing equipment, possibly providing a simple “drop-in” solution using existing factories.
In lab tests, the team demonstrated that the silk-based coating material could be used in existing, standard spray-based manufacturing equipment to make a standard water-soluble microencapsulated herbicide product; it was then tested in a greenhouse on a corn crop. The test suggested it worked better than an existing commercial product, inflicting less damage to the plants, said MIT postdoc Muchun Liu.
By precisely adjusting the polymer chain arrangements of silk materials and adding a surfactant, it’s possible to fine-tune the properties of the resulting coatings once they dry out and harden. The material can be hydrophobic even though it is made and processed in a water solution, hydrophilic, or anywhere in between. For a given application it can be made to match the characteristics of the material it is being used to replace.
“To encapsulate different materials, we have to study how the polymer chains interact and whether they are compatible with different active materials in suspension,” Liu said. The payload material and the coating material are mixed in a solution and then sprayed. As droplets form, the payload tends to be embedded in a shell of the coating material, whether that’s the original synthetic plastic or the new silk material.
The new method can make use of low-grade silk unusable for fabrics — large quantities of which are currently discarded because they have no significant use — and used, discarded silk fabric.
The research team includes Liu, Marelli, Pierre-Eric Millard, Ophelie Zeyons, Henning Urch, Douglas Findley, and Rupert Konradi from the BASF corporation in Germany and in the U.S. The work was supported by BASF through the Northeast Research Alliance.
Here, an exclusive Tech Briefs interview with Marelli. (Edited for length and clarity.)
Tech Briefs: What's the next step in your research with this alternative silk material?
Marelli: For the microplastics, the plan is to keep exploring the microencapsulation — so other types of payloads that we can incorporate, other fabrication methods. We just published another manuscript where we investigated the degradation of silk in soil and a marine environment, so that also allows us to better understand the fate of these microcapsules when they go and are released in nature. We are going to keep working toward this direction.
Tech Briefs: Do you have any idea when such methods will be commercially available?
Marelli: That's hard for me to say. [Being commercially available] means that it can be scaled, so production at scale, it means that the performance is guaranteed with the standards that are required by the industry. And the material needs to be approved to be dispersed in the environment. So that would also require some legislation effort … the plan, of course, is to try to develop as much as we can in the lab, then eventually if a company or an industry wants to get the license and produce in-house, that would be a work goal.
Tech Briefs: What are the pros and cons of this microplastic research?
Marelli: The pros are that we are decreasing the release of microplastics. You substitute the [harmful] material with the material that harmlessly degrades in the environment after it can release the payload. The cons at the moment are that the plastic industry is so well-developed that it is very, very difficult to break in.
You need the type of legislation that projects to ban microplastics by 2025, so that the big companies become interested in using new materials and the materials that are more circular compared to the linear plastics. So, it’s how can we scale up? How do we move silk around? Typically, silk is mostly produced in China [90 percent of silk is produced there], so if we need to use it in other parts of the world, it’s how we sustainably get silk around the world. Then, how can we make enough silk and how can we make enough microparticles with predictable and reproducible properties so that we can really outcompete the current microplastics industry.
Tech Briefs: How do you think this will change or solve the microplastics game?
Marelli: You’re not going to completely change the microplastics game. If you look at microplastics production, the majority of them, for example, come from tires or from the microplastics that you release when you wash your laundry. You cannot replace plastic; plastic is still a pretty big and beautiful material that has a big, positive impact in our life. However, it is becoming negative for the planet. We need to develop and discover new enzymes. We need to discover new microorganism. We need new chemicals that can break down plastics.
At the same time, we have the opportunity to substitute some of these plastics with biopolymers, so materials that originate from natural materials that we can harvest from the environment and upscale to become technical materials. Our duty as academics is to explore these materials, to understand their limit, and to understand how we can push their engineering and design so that we can replace, at least in parts, the microplastics.